M. MELLONI ON THE POLARIZATION OF HEAT. 345 
we look at those lights through our systems of tourmalines with their 
axes first in the parallel and then in the perpendicular position, all the 
coloured images which present themselves with sufficient vividness and 
brilliancy in the first case, completely disappear in the second ; or if any 
of them remains, it is but an excessively feeble gleam*. The rays of 
platina in a state of incandescence, and of copper heated to 400°, repre- 
sent, relatively to radiant heat, the coloured alcohol flames; and the rays 
of a Locatelli lamp, transmitted through water, glass, or alum, are, in 
respect to this heat, no more than the lights of different colours which 
we perceive through the coloured glasses. Now we have seen that the 
effect of the tourmalines on the several species of rays, so far from being 
equal as in the case of light, presents differences so strongly marked, 
that sometimes the heat passes in a sensibly equal quantity in all posi- 
tions of the axes of crystallization, and at others it is almost completely 
intercepted when the axes of the two plates are perpendicular to each 
other. 
The phenomena of calorific polarization, in a variable proportion, 
produced by a system of tourmalines which polarizes the luminous rays 
of all colours equally, are analogous to the extremely marked differences 
of absorption which the various species of calorific rays undergo in their 
passage through a sufficiently thin plate of glass, rock crystal, water, al- 
cohol, and almost all perfectly diaphanous substances, the absorbent 
force of which, within these limits of thickness, if there be any, is the 
same for all sorts of luminous rays+. Among those actions which vary 
* M. Biot possesses a prism cut out of a tourmaline of a light violet-red co- 
lour, which not only does not completely extinguish the common sheaf, as is 
done by the tourmalines that are too thin or too lightly coloured, but colours the 
sheaf while it weakens it ; so that the two images of a minute object seen through 
this prism are sensibly white and of equal intensity near the apex of the refrin- 
gent angle; but, in proportion as the eye moves towards the thickest part, the 
ordinary image is observed to decrease in its intensity and to take at the same 
time a red tint which becomes gradually deeper, while the extraordinary image 
never presents itself in any other colour than a slight tinge of the shade which 
belongs to the tourmaline. It appears then that two flakes of this particular kind 
of tourmaline would not act alike on all the coloured rays, and that in the per- 
pendicular position of the axes they would still be permeable to red light. Per- 
haps the red rays themselves would be finally extinguished if the two flakes 
were of acertain thickness. However, this case is but an exception; for all 
the other tourmalines constantly produce the effect we have just announced ; 
that is to say, that they extinguish indiscriminately and equally all the luminous 
rays whatever be their colour, provided they are made to pass through the flakes 
when their axes are at right angles to each other. 
+ The experiments on the different degrees in which coloured and uncoloured 
media absorb the light and heat of the solar spectrum (Ann. de Chim. et de Phys., 
Dec. 1835, p. 402; Scientific Memoirs, vol. i. part i.) afford a still more striking 
example of the same kind; for the differences then present themselves in pairs 
of calorific and luminous rays, which being isolated and, so to speak, purified by 
_ by the force of refraction, seem to constitute that species of light and heat which 
are most identical. And here I shall take the opportunity to correct a strange 
